An obligate parasite is a parasitic organism that cannot complete its life cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce.
Mycobacterium, genus of rod-shaped bacteria of the
family Mycobacteriaceae (order Actinomycetales), the
most important species of which, M. tuberculosis and
M. leprae, cause tuberculosis and leprosy, respectively,
in humans. M. bovis causes tuberculosis in cattle and in
humans. Some mycobacteria are saprophytes (i.e., they
live on decaying organic matter), and others are
obligate parasites. Most are found in soil and water in a
free-living form or in diseased tissue of animals.
Streptomycin, rifampin, and species-specific
antimicrobial agents have had some success in treating
Mycobacterium infections. Mycobacterium leprae
Mycobacteria are slender rods that sometimes show branching, filamentous
forms resembling fungal mycelium.
The genus Mycobacterium contains three groups:
o Obligate parasites
o Opportinistic pathogens
• Mycobacterium tuberculosis complex
Contains M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti, M. caprae
and M. pinnipedii
• Mycobacterium leprae
Non-tuberculosis mycobacteria (NTM)
This group contains mixed group of isolates from diverse sources: birds, cold-blooded
and warm-blooded animals, from skin ulcers, and from soil, water and other
They are opportunistic pathogens and can cause many types of disease.
Source of infection:
Open case of pulmonary tuberculosis
Mode of infection
Direct inhalation of aerosolized bacilli contained in the droplet nuclei of expectorated sputum.
Infection also occurs infrequently by ingestion for example, through infected milk, and rarely
Transmission of M.tuberculosis
Millions of tubercle bacilli in lungs (mainly in cavities).
Coughing projects droplet nuclei into the air that contain
One cough can release 3,000 droplet nuclei.
One sneeze can release tens of thousands of droplet nuclei
The initial infection with M. tuberculosis is referred to as a
Subsequent disease in a previously sensitized person, either from an exogenous source or
by reactivation of a primary infection is known as
Both forms exhibit quite different pathological features.
Difference b/w Primary and Postprimary
Characteristics Primary Postprimary
Site Any part of
Local lesion Small Large
Cavity formation Rare Frequent
Infectivity* Uncommon Usual
Local spread Uncommon Frequent
Tubercle bacilli do not contain or secrete a toxin.
The exact basis of their virulence is not understood, but
seems to be related to their ability to survive and multiply
Humoral immunity appears to be irrelevant.
The only specific immune mechanism effective is the CMI.
The key cell is the activated CD4+ helper T cell which can develop along two
different paths: The Th1 and Th2 cells
Th1 dependent cytokines activate macrophages, resulting in protective immunity
and containment of the infection
Th2 cytokines induce delayed type hypersensitivity (DTH), tissue destruction and
Type of lesion Specimen
Pulmonary tuberculosis Sputum
Early morning sputum samples should be collected for 3
consecutive days in a sterile container
In case of renal tuberculosis, 3-6 morning urine samples
should be collected
Laryngeal swabs or
Renal tuberculosis Urine
Tuberculosis meningitis CSF
Ziehl-Neelsen staining (hot staining method)
Kinyoun’s method (cold staining method)
Acid fast bacilli resist decolourisation with acid and alcohol
once they have been stained with carbolfuchsin.
AFB appear as pink, long, slender bacilli with beaded
Fluorescent staining by Auramine O or auramine rhodamine
Mycobacterium spp. will fluoresce yellow against dark background under fluorescent microscope
Concentrated specimen is inoculated
on Lowenstein – Jensen’s medium and
incubated at 370C for 2 – 8 weeks
Colonies appear as buff coloured, dry,
irregular colonies with wrinkled surface
and not easily emulsifiable
(Buff, rough and tough colonies)
Colonies are creamy white to yellow colour
with smooth surface and easily emulsifiable
Detection of antibodies
Various methods such as enzyme linked immune sorbent assay (ELISA), radio immunoassay (RIA), latex
agglutination assay have been employed for detection of antibodies in patient serum.
However, diagnostic utility of these methods is doubtful.
WHO has recommended that these tests should not be use for diagnosis of active tuberculosis.